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Downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. The paper was published in the proceedings of the 7th International Conference on Earthquake Geotechnical Engineering and was edited by Francesco Silvestri, Nicola Moraci and Susanna Antonielli. The conference was held in Rome, Italy, 17 – 20 June 2019. Earthquake Geotechnical Engineering for Protection and Development of Environment and Constructions – Silvestri & Moraci (Eds) © 2019 Associazione Geotecnica Italiana, Rome, Italy, ISBN 978-0-367-14328-2 Assessment of yield acceleration of an embankment founded on soils susceptible to liquefaction C. Dai, S. Raathiv & D. Sullivan Coffey, Auckland, New Zealand ABSTRACT: The Waikato Expressway is one of New Zealand Transport Agency’s(NZTA) seven roads of National Significance. Coffey was part of the Hamilton Section City Edge Alli- ance (CEA) engaged by NZTA to prepare a geotechnical design for the new motorway. This included both temporary and permanent design and construction monitoring of embankments, cut slopes, retaining walls and ground improvement works. The project site comprises 22km of motorway. Mangaharakeke gully at the southern Sector 7 is the most challenging geotechnical area of the project. The design includes stabilization and support of 14m batter slopes of 1V:0.85H, fill embankments on loose ground which is subject to high potential liquefaction. The design process was governed by the liquefaction and lateral spreading assessment. However, in the New Zealand geotechnical industry, there are different understandings of yield acceleration assessment and lateral spreading criteria. This sometimes leads to delay in design progress. This paper presents that an iterative analyses approach which was carried out to assess yield acceler- ation using “the most suitable” soil shear strength. This iterative analysis reached a soil residual shear strength at yield Peak Ground Acceleration scenario for the lateral spreading assessment. With this assessment, a combined comprehensive solution was designed and constructed, includ- ing a soil nail wall, dynamic replacement and soldier piles. This paper summarises the key com- ponents in the development of design analysis and outlines innovative solutions to issues of liquefiable cut slope support and loose ground improvement design and construction. 1 INTRODUCTION AND GEOTECHNICAL DESIGN CHALLENGES Steepened batter slopes are necessary at the southern end of Sector 7 to reduce the bridge length for both the Mangaharakeke and Mangaone Bridges. This also avoids encroaching beyond des- ignation line at the crest of the slopes. Ground improvement must be done for the fill embank- ments to the north and south of Mankaharakeke Bridge in Sector 7 between Ch19100 and Ch19207m (also referred as Mangaharakeke Embankment) and between Ch19307 to Ch19351m (also referred as Mangaone Embankment) to achieve the project design criteria for earthworks. There is about 60m cut slope between these two bridges. The assessment of dis- placement by co-seismic, design component diversity and complexity of design solution in a short distance of 60m over loose liquefiable soils are the main design challenges in this area. The design requirements for the Mangaharakeke cut slopes and its support are: • The minimum requirements (MR) specified by the NZTA in Project Alliance Agreement, for earth slopes associated with Express-way Structures are derived and summarised in Table 1 below. • NZTA Bridge Manual, 3rd edition, including amendment 1 (BM). Seismic criteria governed the infrastructure geotechnical design in this project. The seismic design criteria are summarised in Table 1 below. The primary challenge for this project was that some subsoil layers are liquefiable under design seismic condition. The above design cri- teria are essential to access the displacement by co-seismic accurately and to ensure the design component satisfies the MR and BM. The details are discussed in section 3 and 4 below. 1935 Table 1. Seismic design requirements for earthwork Min. Peak Ground Acceleration value Disp. limits Seismic Conditions FOS Soil Strength used in the model (mm) Seismic Ultimate Limit 1.1 Residual 0 <250 State (ULS) Seismic Serviceability 1.0 Residual 65% ≤50 Limit State (SLS) 1.0 Pre-earthquake 100% ≤50 strength In this approximate 60m linear length along the road alignment area, the subject embank- ment sections comprised the following design components. Initial design includes: i. Slope batter of 2H:1V upper slope; ii) 0.84H to 1V (50 degrees) Soil nail cut slopes for the bottom half of the slope above road level and iii) Dynamic replacement around the toe of the embankment slope. Figures 1 and 2 below illustrates the design layout plan and its main components. 2 GEOLOGICAL SETTING AND DESIGN PARAMETERS The Southern Gully Slopes of Mangaharakeke are located within the Lowlands geological ter- rane and the units vary in thickness, generally with no clear trend. The gully floor level between Ch19287.5 to Ch19352m is moderately flat, at 20.5mRL. Recent alluvium (Unit 1b) is obvious the base of the gullies, generally at a thickness of around 3 to 5m. The main geotechnical units with the main geotechnical design components are illustrates in Figure 1 above. The ground conditions comprise Hinuera Formation (Unit 2) overlying Puketoka Forma- tion – Upper Volcanic Source (Unit 4). Puketoka Formation – Lower mixed source (Unit 5), was found underlying the Unit 4. Some of the exploratory holes have reached deep enough to proof the thickness of the Unit 5B and reached Unit 6 – the undifferenti-ate3d Walton Group. The Hinuera Formation (Unit 2) encountered at the site are in the following order: • Unit 2A is a low energy alluvial deposits with localized Lacustrine and peat lenses. • Unit 2B was found beneath the Unit 2A. Unit 2B is a cross stratified flood deposits. • Unit 2C is a low energy alluvial deposit similar to Unit 2A. • Unit 2D was found at the bottom of the Formation. The Unit 2D is similar to Unit 2B. Puketoka Formation - Upper Volcanic Source (Unit 4) found on site are in the following order: Figure 1. Mangaharakeke layout plan and typical cross section with analysis geotechnical model 1936 Figure 2. Flow chart to calculate ay • Unit 4B is an interbedded ashfalls and peat beds, alluvium, completely to highly weathered ignimbrite and Kauroa Ash beds (Walton Subgroup). • Unit 4C was found overlying by the Unit 4B, it generally consists of moderately to slightly weathered non-welded ignimbrite. The project has benefited from a relatively comprehensive site investigation and laboratory testing program. The geotechnical interpretation report (GIR) (Coffey 2016) derived all soil unit design parameter, which were listed in the report. Geotechnical design analysis in Sec- tion 4 indicated that Unit 1 and certain thickness of Unit 2 and 4 layers will be liquefied under design seismic load, and the shear strength of liquefied layer will depend on the vertical stress ratio from liquefaction analysis. The details are summarized in Section 4. 3 RATIONAL LIQUEFACTION AND YIELD ACCELERATION ASSESSMENT One of the key assessment criteria is the Displacement by Newmark’s sliding block approach. A yield acceleration (ay) estimate is required to assess this co-seismic displacement. The accel- eration at which movement occurs is the yield acceleration. It is defined as that acceleration that produces a pseudo static global stability Factor of Safety (FOS) of 1. The acceleration that produces FOS = 1 depends on both geometry of the slip surface, and the shear strength of the soil. Clearly, limit equilibrium slope stability analysis programs such as Slope W or Slide can investigate a wide range of potential slip surface geometries (circle and non-circle) once an appropriate soil shear strength has been determined. However, what that “appropri- ate” soil strength is not clear in the New Zealand geotechnical industry. There are several opinions; for example, should we adopt: Opinion 1: Use the liquefied residual soil shear strength (LRSSS) determined by the ay seismic condition which may not affect all potentially liquefiable material which is identified during extreme (ULS or Maximum Considered Earthquake (MCE)) seismic event; or Opinion 2: Use the LRSSS determined by the ULS or MCE event which is likely to liquefy all the potentially at-risk soils beneath the site. Opinion 1 is correct according to the author’s view because it accounts for the most likely design scenario to affect the site over the life of the project, and because of a recommenda- tion in the BM: the displacement likely at the design event seismic response associated with slope collapse avoidance, shall be assessed using moderately conservative soil strengths consist- ent with the anticipated stress-strain behavior and relevant strain levels and a Newmark sliding 1937 block displacement. However, so called moderately conservative strengths are not clearly defined. On this basis, the “appropriate” soil shear strength to be used to assess Newmark’s sliding block analysis should be determined using the following iterative approach: 1. Non- Liquefiable
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